The present invention is directed to a fuel injection system for an internal combustion engine and more particularly to a fuel injection system for an engine which includes a vortex flow meter for measuring the flow rate of the intake air for the engine, a fuel injection valve adapted to be driven in synchronism with the frequency output signal of the vortex flow meter to thereby inject a predetermined amount of fuel and means for controlling the pressure of the fuel to be fed to the fuel injection valve in accordance with the running state of the internal combustion engine.
As is well known in the art, a vortex flow meter generates an output signal having a frequency which corresponds to the vortex velocity (or the velocity of a number of vortices generated) which is proportional to the flow rate of intake air to be metered. The method of supplying a predetermined quantity of fuel to the engine by injection in synchronism with a frequency output signal proportional to the flow rate of the intake air is disclosed in laid-open Japanese Utility Model No. 53-133919 or laid-open Japanese Patent Publication No. 55-5448. According to these disclosures, it is possible to inject the fuel in accordance with an abrupt change in the intake air flow rate during the deceleration of the engine so that the engine has an advantage in that the accelerating performance is excellent. The intake air flow rate of an engine having a displacement of 2,000 cc is about 2 l/s (during the idling operation) to 85 l/s (at the maximum output). On the other hand, the torque fluctuates unless the injection frequency of the fuel injection valve is higher than at least about 10 Hz during the idling operation. As a result, during high speed running operation of the engine, the injection frequency of the aforementioned fuel injection valve necessarily becomes high. Since the injection frequency is high, the period for the injection valve to be opened can not be elongated. This is especially true for the conventional method of compensating the flow rate of the fuel to be fed to the engine by changing the aforementioned open period for the valve in accordance with the running state of the engine wherein the valve open period is remarkably shortened thereby creating a defect in that the metering error of the fuel for a short period is augmented.